Control of heating in active doped optical fiber

ABSTRACT

Herein is provided a fiber length including a doped fiber core extending over the fiber length. First and second cladding regions radially surround the core. At least one pump light input site is arranged to accept input pump light into the first cladding region. A low-absorption length over which the first cladding region has a first cross-sectional geometry produces a first level of absorption of input pump light from the first cladding region to the core, extending from a pump light input site for an extent over which the doped core can absorb at least about 10% of input pump light from the first cladding region. A high-absorption section over which the first cladding region has a second cross-sectional geometry produces a second level of absorption of input pump light from the first cladding region to the core, greater than the first level of absorption of input pump light.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 62/448,017, filed Jan. 19, 2017, the entirety of whichis hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Contract No.FA8721-05-C-0002, awarded by the U.S. Air Force. The Government hascertain rights in the invention.

BACKGROUND

This invention relates generally to optical fiber, and more particularlyrelates to active, doped optical fiber.

The development of doped, actively-pumped optical fiber has beenfundamental for enabling the application of optical fiber systems, suchas fiber-based laser systems, to the fields of telecommunications,sensor systems, and electro-optic systems, as well as for industrialapplications such as high-speed material cutting, welding, andmicromachining, among other applications. In one example, a doped,actively-pumped optical fiber generally consists of a fiber core regionincluding a core material that is doped with a selected dopant forcausing lasing of the core material at a selected wavelength. The fibercore region is surrounded by a first cladding region though which pumplight is introduced to the fiber, e.g., from one or both ends of thefiber. The pump light in the first cladding region is absorbed by thecore region along the fiber length, causing population inversion andlasing of the material in the core region, with the core region carryingthe laser light through the fiber. The first cladding region istypically surrounded by a second cladding region that is provided as anouter protective layer for the fiber and to confine the pump light tothe first cladding and core regions.

In operation, the core region of a doped, actively-pumped optical fibergenerates heat as the input pump signal is converted to a laser signal,and this heating primarily occurs in the core material, due to, e.g.,quantum defects of the lasing process and the degree of pump and signalabsorption. Because the core region generally has a very largesurface-to-volume ratio in a conventional fiber geometry, the heatgenerated in the core region dissipates to the outer cladding regions ofthe fiber. This heat generation and dissipation can result in thermaldamage of the fiber core material as well as the materials of both thefirst fiber cladding and second fiber cladding. Heat generation anddissipation can also cause detrimental thermo-optical effects duringfiber laser operation, such as multi-mode instability, also referred toas transverse mode instability and modal instability, of the lasersignal in the core of the fiber.

Conventionally, a lasing pump signal is introduced into a doped,actively-pumped fiber system through the first cladding layer of thefiber, from one or both ends of the fiber, with the pump light crossinginto the fiber core as the pump light progresses down the length of thefiber. In the doped fiber core, the pump energy is absorbed andconverted to the target laser wavelength. The geometry and materialcomposition of the fiber core region and the fiber cladding regions aregenerally homogeneous along substantially the entire length of thefiber, resulting in nonuniform, with respect to length, distribution ofpump energy and correspondingly non uniform heat dissipation along thefiber length. The magnitude of this absorption and heating depends onthe doping and other characteristics of the fiber materials as well asthe choice of pump and target laser wavelengths. The heat that isgenerated in the doped fiber core occurs at regions of highest signalpower and highest gain, i.e., the site of highest conversion of pump tosignal power. This is in general relatively near to locations along thelength of the doped fiber where pump power is introduced into the firstcladding region of the fiber. Thus, there can be regions of fiber alongwhich heat generation is pronounced, and there can exist specific sitesalong the fiber length at which heat generation is extreme.

Many fiber laser applications, e.g., multi-kW fiber lasers and multi-Wamplifiers for industrial material processing and telecommunications,require robust and sustained fiber operation over long durations. But asa result of heating during fiber laser operation, the requirements ofmany such applications are not attainable with conventional fibersystems. As a result, the design and system limitations imposed by theneed to prohibit thermal damage of fiber lasers prohibits the ability toachieve the reliable optical power scaling required by many importantfiber applications.

SUMMARY

Herein is provided a fiber that overcomes the limitations that areconventionally imposed by the need to prohibit thermal damage of a fiberduring high-power fiber operation. The fiber provides a continuous fiberlength that includes a doped fiber core extending over the continuousfiber length. A first cladding region radially surrounds the doped fibercore over the fiber length. A second cladding region radially surroundsthe first cladding region over the continuous fiber length. At least onepump light input site is arranged to accept input pump light into thefirst cladding region.

The continuous fiber length includes a low-absorption length over whichthe first cladding region has a first cross-sectional geometry thatproduces a first level of absorption of input pump light from the firstcladding region to the doped fiber core. This low-absorption lengthextends along the fiber length from a pump light input site for anextent over which the doped fiber core can absorb at least about 10% ofinput pump light from the first cladding region. The continuous fiberlength also includes a high-absorption section of the fiber length overwhich the first cladding region has a second cross-sectional geometrythat produces a second level of absorption of input pump light from thefirst cladding region to the doped fiber core. This second level ofabsorption of input pump light is greater than the first level ofabsorption of input pump light.

This fiber provides an ability to achieve implementation of multi-kWfiber lasers and multi-W amplifiers for modern fiber applications.Specifically, the reduction in heat that this design enables providesfor superior operation of pulsed fiber lasers and amplifiers in whichthe fiber length is preferably as short as possible. Applications inmaterials processing, medicine, e.g., laser surgery, LIDAR,communications, and other fields are enabled by the design. In addition,single mode lasers and amplifiers, pulsed Quasi-CW, and CW, all in whichit is preferable to minimize fiber length, are also well-addressed.Material processing applications are also well addressed, for example,for high power, Yb-doped industrial lasers and amplifiers, oftenexceeding 1 kW in average power, and for high power lasers andamplifiers doped with other dopants, such as erbium. In all of theseapplications, the reduction in heat dissipation achieved by the fiberdesign provided herein enables fiber operation that far surpasses thatof conventional fiber configurations. Other features and advantages ofthe fiber will be apparent from the following description andaccompanying figures, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional side view of a fiber including a core,first cladding region having a cross-sectional geometry that is changedover the length of the fiber, and a second cladding region;

FIGS. 1B-1C are cross-sectional radial views of the fiber of FIG. 1Ataken at a site along the fiber at which the first cladding region has acircular cross-sectional geometry and taken at a site along the fiber atwhich the first cladding region has an octagonal cross-sectionalgeometry, respectively;

FIGS. 2A-2E are schematic cross-sectional views of example pairs ofcross-sectional geometries of a fiber first cladding region betweenwhich the cross-sectional geometry of the fiber first cladding regioncan shift;

FIG. 3 is a plot of pump and signal power in a fiber as a function ofposition along the fiber length for an end-pumped, co-propagating fiberamplifier with an octagonal first cladding region cross sectionalgeometry along the entire fiber length;

FIG. 4 is a plot of pump and signal power in a fiber as a function ofposition along the fiber length for an end-pumped, counter-propagatingfiber amplifier with an octagonal first cladding region cross sectionalgeometry along the entire fiber length;

FIG. 5 is a plot of pump and signal power and heat deposited in a fiberas a function of position along the fiber length for an end-pumped,co-propagating fiber amplifier with an octagonal first cladding regioncross sectional geometry along the entire fiber length;

FIG. 6 is a plot of pump and signal power and heat deposited in a fiberas a function of position along the fiber length for an end-pumped,co-propagating fiber amplifier with a circular first cladding regioncross sectional geometry along the entire fiber length;

FIG. 7 is a plot of pump and signal power and heat deposited in a fiberas a function of position along the fiber length for an end-pumped,co-propagating fiber amplifier with a circular first cladding regioncross sectional geometry along a fiber length up until 6 m and anoctagonal first cladding region cross-sectional geometry for theremainder of the length;

FIG. 8 is plot of the peak heat dissipated along a portion of a fiberhaving a circular cross-sectional cladding, for a different site, inmeters, along an effective fiber length at which the first claddingregion cross section changes from circular to octagonal, and each datapoint in the second line represents the peak heat dissipated along aportion of a fiber having an octagonal cross-sectional cladding, for adifferent site, in meters, along an effective fiber length at which thefirst cladding region cross section changes from circular to octagonal;

FIG. 9 is a plot of the pump light power, the signal light power, andthe heat deposited in a for a fiber having a change in cross-sectionalgeometry of the first cladding region from circular to octagonal at theposition where 5% of the pump power has been absorbed, which is 1.0meters;

FIG. 10 is a plot of the pump light power, the signal light power, andthe heat deposited in a for a fiber having a gradual change incross-sectional geometry of the first cladding region from circular tooctagonal between 3.5 m and 7.5 m from a first end of the doped fiber;

FIG. 11 is a plot of the pump light power, the signal light power, andthe heat deposited in a for a fiber having a change in cross-sectionalgeometry of the first cladding region from circular to octagonal at theposition where 20% of the pump power has been absorbed, which is 2.0meters;

FIG. 12 is a plot of the pump light power, the signal light power, andthe heat deposited in a for a fiber having a change in cross-sectionalgeometry of the first cladding region from circular to octagonal atposition where 30% of the pump power has been absorbed, which is 3.0meters;

FIGS. 13A-13E are schematic views of fiber amplifier configurations thatinclude both undoped and doped fiber lengths; and

FIGS. 14A-14B are schematic views of a fiber draw tower that includesshaping hardware and control systems for shaping the geometry of thefirst cladding region of a fiber.

DETAILED DESCRIPTION

Referring to FIGS. 1A, 1B, and 1C, an active doped fiber 10 providedherein has a length, L, between two ends of the fiber 10 and includes adoped fiber core 12, in which signal light propagates and through whichsignal light is delivered between the ends of the fiber. The fiber core12 is surrounded radially by a first cladding region 14 having a firstcross-sectional geometry 16 along one or more portions of the fiberlength. The first cladding region has a second cross sectional geometry18 that is different from the first cross-sectional geometry 16 alongone or more portions of the fiber length. Additional different crosssectional geometries of the first cladding region can be included alongone or more portions of the fiber length. Thus, in one embodiment, thecross-sectional geometry of the first cladding region is changed betweenat least two different cross-sectional geometries at a site 27 along thelength of the fiber. This change can be abrupt or can be gradual. In oneembodiment, the cross-sectional geometry of the first cladding region isgradually changed between two different cross-sectional geometriesthrough a transition extent along the length of the fiber. In analternate embodiment, the cross-sectional geometry of the first claddingregion is abruptly changed between two different cross-sectionalgeometries. Both of these embodiments result in the imposition of atleast two different cross-sectional geometries along the length of thefiber.

A second, outer cladding region 20 radially surrounds the first claddingregion and provides an outer protective layer for the fiber. The secondcladding region material confines the pump light to the first claddingregion and the fiber core. The second cladding region and the firstcladding region can be provided as the same material or as differentmaterials. The radial cross-sectional geometry of the second claddingregion is substantially independent of that of the first claddingregion. In one embodiment, the outer diameter of the second claddingregion is substantially constant along the fiber length. Thisconstant-diameter second cladding region can be, e.g., circular, orother suitable geometry.

Pump light 22 is introduced to the fiber at one or more pump input sitesof the first cladding region 14, such that the pump light is input tothe first cladding region 14 for absorption of the pump light into thefiber core region from the first cladding region. In one embodiment, apump input site 25 is arranged at an end of the fiber. Pump input lightcan also be introduced to the first cladding region at pump input sitesalong the fiber length in the manner described below. Wherever the pumplight is introduced into the first cladding region, the cross-sectionalgeometry of the first cladding region affects the degree of absorptionof the pump light into the doped fiber core region from the firstcladding region. The first cross-sectional geometry 16 of the firstcladding region is defined as a cross-sectional geometry that produces afirst level of absorption of pump light from the first cladding regioninto the core, and therefore that produces a first rate of conversion ofpump light to signal light per unit length of the fiber. The secondcross-sectional geometry 18 of the first cladding region is defined as across-sectional geometry that produces a second level of absorption ofpump light from the first cladding region into the fiber core, andtherefor that produces a second rate of conversion of pump light tosignal light. In one embodiment, the second level of absorption of pumplight into the fiber core is greater than the first level of absorptionof pump light into the fiber core. In an alternative embodiment thesecond level of absorption of pump light into the fiber core is lessthan the first level of absorption of pump light into the fiber core.

The first and second cross-sectional geometries of the first claddinglayer are selected to produce the two different levels of absorption ofpump light into the fiber core. The two first cladding cross-sectionalgeometries 16, 18 shown in FIG. 1B and FIG. 1C are meant as examplesonly and are not limiting, and as explained below, other cross-sectionalgeometries can be employed as-suitable. And as stated above, the firstfiber cladding region 14 of the fiber 10 can include more than twodifferent cross-sectional geometries along the fiber length, and cantransition between geometries either abruptly or gradually. A range ofexample cross-sectional geometries are described below. Whatevercross-sectional geometries are selected for the first cladding region ofthe fiber, at least one of the cross-sectional geometries limits theabsorption of pump light into the fiber core to a larger degree than theother or others of the cross-sectional geometries.

Conventionally, a first cladding region having a cross-sectionalgeometry that includes one more generally flat sections, and that isgenerally symmetric, as in the octagonal cross-sectional geometry 18shown in FIG. 1C, tends to enhance the absorption of pump light from thefirst cladding region to the fiber core. This geometry promotescontinual mixing of pump light in the first cladding region and promotesefficient crossing of the light into and through the fiber core regionfor absorption by the core region. Conversely, a first cladding regionhaving a generally rounded cross-sectional geometry, as in thecross-sectional geometry 16 shown in FIG. 1B, tends to limit absorptionof pump light from the first cladding region to the fiber core becausewith this geometry, the degree of core crossing of the light is reduced.

As pump light is absorbed into the fiber core and converted in the coreto signal light, heat is produced in the core and dissipated throughoutthe fiber, and this heat increases to a peak temperature along the fiberlength for at least some portion of the fiber length. Heating of thefiber due to dissipation of heat that is generated internal to the fiberposes a range of limitations for fiber operation. A reduction in peakfiber temperature is therefore desirable for fiber amplifier and fiberlaser systems to combat the issues that limit power scaling and theincreased heat such can cause, such as damage/failure of the acrylate,or other, outer, second cladding material of the fiber; the cost andcomplexity of localized fiber cooling scenarios, and multimodeinstability (MMI) that is caused by increased fiber core temperatures.

In the fiber 10 provided herein, the cross-sectional geometry of thefirst cladding region cross-sectional geometry is controlled along thefiber length to cause reduced pump light absorption at one or more sitesor along one or more portions of the fiber length so that the heatgenerated at the one or more sites or along the one or more portions isreduced from that which would otherwise be produced by a cross sectionalgeometry that would be more favorable to pump light absorption by thefiber core. In the example cross-sectional geometries shown in FIG.1B-1C, the pump light absorption efficiency per unit length for theoctagonal geometry 18 is about twice the pump light absorptionefficiency per length for the circular geometry 16 when the pump lightis introduced from a pump input site in the first cladding region.Similar differences in pump light absorption efficiency are achievedbetween pentagonal, hexagonal, spiral, D-shaped, and othercross-sectional geometries, compared with a circular geometry.Therefore, the sites along or portions of a fiber length at which thefirst cladding region has a circular cross-sectional geometry has lesspump light absorption, and less heating, than the sites along orportions of a fiber length at which the first cladding region has across-sectional geometry that is non-circular, e.g., including one ormore flats around the periphery of the region. In the example embodimentof FIGS. 1A-1C, a low-absorption length 33 along the fiber starting fromthe end pump input site has a circular cross-sectional geometry that isrelatively low-absorbing, while a high-absorption length 35 along thefiber, past the transition site 27, has a non-circular cross-sectionalgeometry that is relatively high-absorbing.

No particular cross-sectional geometry, and no particular number ofdifferent cross-sectional geometries, is required for the first claddinglayer of the fiber. All that is required is at least two differentcross-sectional geometries, one of which limits the degree of pump lightabsorption into the fiber core from the first cladding layer more thanthe other cross-sectional geometry. FIGS. 2A-E illustrate examples offirst cladding region cross-sectional geometries that control pump lightabsorption from the first cladding layer to the fiber core, to differingdegrees. For clarity, the doped fiber core 12 and the second claddingregion 20 are not shown, but it is to be understood that the doped fibercore and second cladding region are present in all examples.

In one embodiment, shown in FIG. 2A, the cross-sectional geometry of thefirst cladding region includes a low-absorption cross-sectional geometry30 that is generally circular and a high-absorption cross-sectionalgeometry 32 that is one of octagonal, hexagonal, and pentagonal. In afurther embodiment, shown in FIG. 2B, the cross-sectional geometry ofthe first cladding region includes a low-absorption cross-sectionalgeometry 30 that is generally circular and a high-absorptioncross-sectional geometry 34 that is also generally circular but thatfurther includes one or more substantially flat regions 39 around thecircumference of the geometry. In a further embodiment, shown in FIG.2C, the cross-sectional geometry of the first cladding region includes alow-absorption cross-sectional geometry 30 that is generally circularand a high-absorption cross-sectional geometry 36 that is also generallycircular but that further includes at least one opening, such as a hole38, in the cross section. In a further embodiment, shown in FIG. 2D, thecross-sectional geometry of the first cladding region includes alow-absorption cross-sectional geometry 30 that is generally circularand a high-absorption cross-sectional geometry 40 that is also generallycircular but that further includes at least one perimeter asperity, suchas a perimeter notch 42, at one or more sites around the perimeter.

The transition between two or more cross-sectional geometries can beabrupt, as in FIG. 2A, or can be gradual, as shown in FIG. 2E. In thisfurther embodiment, the cross-sectional geometry of the first claddingregion includes a low-absorption cross-sectional geometry 30 that isgenerally circular and two or more intermediate cross-sectionalgeometries 44 before a final high-absorption cross-sectional geometry 46in the transition. In further embodiments, the cross-sectional geometryof the first cladding region includes a first cross-sectional geometrythat is not circular, e.g., that is one of the high-absorptiongeometries 32, 34, 36, 40, 44, 46 in FIGS. 2A-2E, and a secondcross-sectional geometry that is a generally circular, low-absorptiongeometry.

In one embodiment, the cross-sectional geometry of the first claddingregion is changed from a first, high pump light-absorption geometry to asecond, low pump light-absorption geometry, at one or more locations orportions of the fiber length. This reduces the W/m fiber heating thatwould otherwise occur without use of the low pump signal-absorptiongeometry. In an alternate embodiment, the cross-sectional geometry ofthe first cladding region is changed from a first, low pumplight-absorption geometry to a second, high pump light-absorptiongeometry, at one or more locations or portions of the fiber length, toreduce the W/m fiber heating everywhere except the locations of the highpump light-absorption geometry.

The location of the change in cross-sectional geometry of the firstcladding region, between two different geometries, is preferablydetermined based on the rate and degree of absorption of input pumplight into the doped fiber core. The fiber core material can be anysuitable material, such as a silica-based glass or a fluoride-basedglass, and can be doped with, e.g., ytterbium, erbium, praseodymium,neodymium, and tellurium. This core material and dopant sets theabsorption properties of the core. The choice of the doped length, whichis referred to as the active fiber length for a fiber amplifier or fiberlaser, is influenced by many variables. Some of the main criteriainclude, e.g., cost of the fiber, $/meter and optical non-linearities inthe fiber, which generally scale non-linearly with the length of thefiber. Most optical nonlinearities, such as stimulated Brillouinscattering (SBS), stimulated Raman scattering (SRS), and four-wavemixing (FWM), are unwelcome, and as a result there is often an upperlimit on fiber length to avoid or minimize such. Conversely, pump lightabsorption and conversion of pump power to signal power, scale with thelength of the fiber; if the fiber is too short then a laser or amplifierin which the fiber is implemented is inefficient. Signal loss and signalre-absorption are also important because at very long fiber lengths thesignal stops being amplified and the additional fiber length causessignal loss due to attenuation of signal and also due to re-absorptionof the signal by the doped fiber. This places an upper limit on theuseful length of the doped fiber of a gain stage in a fiber laser oramplifier. In addition, the choice of the pump light wavelength orwavelengths to be injected into the first cladding region, the pumplight power, as well as the signal power and signal wavelength orwavelengths, determine the rate of pump absorption and signal gain.Taking all the above criteria into account, there are generally accepteddesign criteria that can be employed in the conventional manner toselect a doped fiber length for maximized, or close to maximum,efficiency in converting pump energy to signal energy for a given fiberapplication.

In embodiments provided herein, the extent of each cross-sectionalgeometry that is imposed on the first cladding region along the fiberlength is determined based on the percentage of input pump lightabsorption into the fiber core that is prescribed to be achieved alongthat extent. In general, in this analysis, the percentage of input pumplight absorption into the fiber core is calculated as a function ofposition along the fiber length to make this determination. Input pumplight absorption can be calculated using commercially-available fiberamplifier/laser modeling software, or general mathematical software thatis programmed to calculate optical effects, as will be understood bythose skilled in the art, for selected power and wavelengthspecifications. For example, pump light absorption can be modeled andanalyzed with the modeling software RP fiber Power, from RP PhotonicsConsulting, GmbH, Waldstr 17, 78073 Bad Durrheim, Deutschland; with themodeling software OptiSystem 14.2, from Optiwave Systems, Inc., Ottawa,ON, Canada; or with the modeling software RSoft from Synopsys, Inc.,Mountain View, Calif.; or with the mathematical software MatLab, fromthe MathWorks, Natick, Mass.

Alternatively, input pump light absorption into the fiber core can beexperimentally measured. Here, the power at a source of input pump lightis first measured, e.g., with an optical power meter, and then the pumplight is introduced to the doped fiber at an input pump site, such as anend of the fiber, into the first cladding region. At the far end of thefiber, where residual, i.e., un-absorbed, pump light power is output, anexternal filter or other optical element is arranged to blockwavelength(s) other than those of the pump reaching an optical powermeter at end of the fiber. The residual, un-absorbed pump power at thefar end can then be measured. The percentage of pump light absorption isthen given as a percentage as 100×(1-[Output pump power/Input pumppower]).

FIG. 3 is a plot of pump and signal light power along an end-pumped,co-propagating fiber, i.e., a fiber with co-propagating pump and signallight, configured as a fiber amplifier. The power is plotted as afunction of position along the fiber length. The fiber includes a firstcladding region that is octagonal in cross-section as in FIG. 1C, alongthe entire fiber length, or 100% of the fiber length. This plot refersto conditions in which the pump light is launched into the firstcladding region with a pump power of 2000 W at a wavelength of 976 nmand with an input signal power of 10 W at a wavelength of 1064 nm. Thefiber has a core that is ytterbium-doped along its length, having firstcladding pump absorption coefficient of 0.46 dB/meter at 915 nm, with a25 gm-diameter core, and a 400 gm-diameter first cladding region.

This plot provides a determination of input pump light absorption alonga selected extent of fiber length. For example, as shown in the plot,95% of the input pump light, i.e., all input pump light except for about100 W of the input pump light, is absorbed into the fiber core withinabout 10 m along the fiber length. As shown in the plot, 10% of the pumppower is absorbed within about 1 m along the fiber length; the inputpump power is reduced from a starting power of 2000 W at the 0 mposition to about 1800 W at about the 1 m position. Thus, there can bedetermined what percentage of input pump power is absorbed by the fibercore for a given set of fiber conditions, here for an all-octagonalfirst cladding geometry.

FIG. 4 is a plot of pump and signal light power along an end-pumped,counter-propagating fiber amplifier as a function of doped fiber length.The fiber includes a first cladding region that is octagonal incross-section as in FIG. 1C, along the entire fiber length, or 100% ofthe fiber length. This plot refers to conditions in which the input pumplight is launched into the first cladding region with a pump power of2000 W at a wavelength of 976 nm with an input signal power of 10 W at awavelength of 1064 nm. The fiber has a core that is ytterbium-doped,having first cladding pump absorption coefficient of 0.46 dB/meter at915 nm, with a 25 μm-diameter core, and a 400 μm-diameter first claddingregion. As shown in the plot, 10% of the pump power is absorbed withinabout 0.5 m along the fiber length; the input pump power is reduced froma starting power of 2000 W at the 20 m position to about 1800 W at aboutthe 19.5 m position along the fiber length.

Now turning to analysis for determining the preferred location andextent of low pump-absorption geometry for the first cladding regionalong a doped fiber length, consider an example of a fiber including aytterbium-doped fiber core operated with co-propagating pump and signallight and having an octagonal cross-sectional geometry for the firstcladding region, as in FIG. 1C, along the entire fiber length, or 100%of the fiber length. FIG. 5 is a plot of the pump light power, thesignal light power, and the heat deposited in the fiber from the core,along the fiber length, for this fiber arrangement. This plot refers toconditions in which input pump light is launched into the first claddingregion with a pump power of 2000 W at a wavelength of 976 nm and aninput signal power of 10 W at a wavelength of 1064 nm. The fiber has acore that is ytterbium-doped, having a first cladding pump absorptioncoefficient of 0.46 dB/at 915 nm, with a 25 μm-diameter core, and a 400μm-diameter first cladding region. As shown in the plot, 10% of the pumppower is absorbed within about 1 m along the fiber length, that is, 200W of the input pump power of 2000 W is absorbed within about 1 m,reducing the pump power to 1800 W at about 1 m along the fiber length.Also as shown in the plot, the peak fiber heating that occurs in thefiber is about 38 W/m, referring to the right hand vertical axis of theplot, occurring at a little less than 2 m from the start of the fiberlength.

Now consider a fiber including a ytterbium-doped fiber core operatedwith co-propagating pump and signal light and having a circularcross-sectional geometry, rather than octagonal geometry, for the firstcladding region, as in FIG. 1B, along the entire fiber length, or 100%of the length. FIG. 6 is a plot of the pump light power, the signallight power, and the heat deposited in the fiber from the core, alongthe fiber length for this fiber. This plot refers to conditions in whichinput pump light is launched into the first cladding region with a pumppower of 2000 W at a wavelength of 976 nm and an input signal power of10 W at a wavelength of 1064 nm. The fiber has a core that isytterbium-doped, having a first cladding absorption coefficient of about0.23 dB/meter at 915 nm, with a 25 μm-diameter core, and a 400μm-diameter first cladding region. As shown in the plot, 10% of theinput pump power is absorbed within about 1.3 m along the fiber length;that is, about 200 W of the input power of 2000 W is absorbed by about1.3 m along the fiber length, reducing the residual pump power to 1800 Wat 1.3 m along the fiber length. Also shown in the plot, the peak fiberheating is about 24 W/m, occurring at about 1.5 m from the start of thefiber length.

Comparing the fiber absorption shown in the plot of FIG. 5 with thefiber absorption shown in the plot of FIG. 6, it is found that to obtaina selected input pump light absorption and output signal power, a fiberhaving a circular first cladding region cross section must be abouttwice as long as a fiber having an octagonal first cladding region crosssection, both along the entire fiber length. For many applications therequired added length for a circular first cladding region is non-idealbecause additional fiber length generally costs more, making the fiberproduct more expensive, and further because additional fiber lengthincreases the effective length for optical non-linearities, which cancause power scaling limits and/or reduce optical performance.

Considering these factors, in one embodiment of the fiber providedherein, there is employed a low pump-absorption geometry, such as acircular geometry, for the first cladding region, extending along alow-absorption length of the fiber from at or near to a pump input siteto an extent over which at which at least about 10% of input pump power,i.e., 10% of the input pump light, can be absorbed from the firstcladding region to the doped fiber core measured from the fiber end orother fiber site at which the input pump light is launched. For example,if 10% of the input pump light power is absorbed in the first 1 m offiber from where the input pump light is launched, then a circular firstcladding region is imposed along the same 1 m of the fiber length,beginning where the pump light is launched. This configuration producesa noticeable and useful reduction of the peak heating (W/m) in thefiber.

In a related embodiment of the fiber provided herein, there is employeda low pump-absorption geometry, such as a circular geometry, for thefirst cladding region, along a low-absorption length along the fiberhaving an extent over which at least about 15% of input pump light canbe absorbed into the doped fiber core, measured from at or near to thefiber end or other fiber site at which the input pump light is launchedinto the fiber. In a further embodiment, there is employed a lowpump-absorption geometry, such as a circular geometry, for the firstcladding region, along a low-absorption fiber extent having a lengthover which about 20% of input pump light can be absorbed into the dopedfiber core, measured from the fiber end or other fiber site at which theinput pump light is launched into the doped fiber.

In a further embodiment, a low pump absorption-geometry of the firstcladding region extends along a section of the fiber over which leastabout 30% of input pump light can be absorbed, or can extend along afiber portion having a length over which at least about 40% of inputpump light can be absorbed, measured from at or near to the fiber end orother fiber site at which the input pump light is launched into thedoped fiber. In a further embodiment, a low pump-absorption geometry forthe first cladding region is employed over a section of the fiberlength, at or near to a pump light input, along an extent over whichbetween about 10% and about 40% of input pump light can be absorbed fromthe first cladding region into the doped fiber core.

In a further embodiment, a reduced pump-absorption cross-sectionalgeometry, such as a circular cross-sectional geometry, is imposed on thefirst cladding region starting at or near to the fiber end or other sitealong the fiber that is known to be the site of highest un-absorbedinput pump power, and continuing in the direction of travel of the pumplight for an extent over which at least about 10% of the pump light canbe absorbed from the first cladding region to the fiber core. In afurther embodiment, a reduced pump-absorption cross-sectional geometry,such as a circular cross-sectional geometry, is imposed on the firstcladding region starting at the site along the fiber after which about30% of input pump light can be absorbed into the fiber core from thefirst cladding region, and continuing in the direction of travel of thepump light as a low-absorption length, for an extent over which no morethan about 80% of the pump light can be absorbed from the first claddingregion to the fiber core, or for an extent over which no more than about90% of the pump light can be absorbed from the first cladding region tothe fiber core. The low-absorption length thereby extends from a site atwhich 30% of has been absorbed to a site at which about 80% or 90% ofinput pump light is absorbed into the doped fiber core.

For implementations in which the pump light is injected at sites alongthe length of the fiber at one or more non-end positions, each fiberinterval between pumps is to be considered separately. The site alongthe fiber at which the cross-sectional geometry change starts is hereindefined as the site at which the geometry begins to change gradually orchanges abruptly.

In further embodiments, the reduced pump-absorption cross-sectionalgeometry is imposed on the first cladding region starting at the fiberend or other site along the fiber that is known to be the site forhighest un-absorbed input pump power, and continuing in the direction oftravel of the pump light for an extent over which at least about 20% ofthe pump power can be absorbed from the first cladding region to thefiber core; for an extent over which at least about 30% of the pumppower can be absorbed from the first cladding region to the fiber core,for an extent over which at least about 40% of the pump power can beabsorbed from the first cladding region into the fiber core, or for anextent over which at least about 50% of the pump power can be absorbedfrom the first cladding region to the fiber core.

Now considering one example of this embodiment, a 12.5 m-long effectivedoped length of fiber is configured with co-propagating pump light inthe first cladding region and signal light in the doped fiber core. Theinput pump light is launched into the first cladding region with a pumppower of 2000 W at a wavelength of 976 nm and an input signal power of10 W at a wavelength of 1064 nm. The fiber has a core that isytterbium-doped, at about the same concentration across the entirelength of the 12.5 m fiber with a 25 μm-diameter core. FIG. 7 is a plotof the pump light power, the signal light power, and the heat depositedin the fiber from the core, along the fiber length, for this fiberconfiguration.

Turning to FIG. 7, this example fiber has two differing first claddingregion cross-sectional geometries across the length of the fiber, asshown in FIG. 7. Along the fiber length from one end, at 0.0 m, to asite at 6.0 m, i.e., from the input end to 6.0 m along the fiber length,the first cladding region is shaped with a low-absorption circularcross-sectional geometry. This geometry has a first cladding absorptioncoefficient of about 0.23 dB/meter at 915 nm, with an effective dopedfiber length for about 95% pump absorption of 20 m, based on theanalysis given above and shown in the FIG. 6. At fiber positions between6.0 m and 12.5 m the first cladding region is shaped with ahigh-absorption octagonal cross-sectional geometry. This geometry has afirst cladding absorption coefficient of about 0.46 dB/meter at 915 nman effective doped fiber length for about 95% pump absorption of 10 m,based on the analysis given above and shown in the FIG. 5. At the fiberposition of 6 m about 57% of the pump power has been absorbed; here the2000 W input pump power is reduced to about 860 W.

In this embodiment, for about 50% of the fiber length, the firstcladding region is shaped with the low-absorption circular geometry toreduce heat dissipation in the fiber; 6 m/12.5 m of the fiber length.For each of the two geometries, the first cladding region is 400 μm indiameter. As shown in the plot, 95% of the pump light, i.e., all inputpump light except for about 100 W, is absorbed into the fiber corewithin about 12.5 m of fiber length. The peak fiber heating is about 27W/m. This is significantly less than the 38 W/m demonstrated in theall-octagonal cladding fiber, the results of which were shown in FIG. 5above, and this is only slightly greater than the 24 W/m heatingproduced in an all-circular first cladding region.

In this example the site of the cross-sectional geometry change is at6.0 m of the 12.5 m total length of this design, which also correspondsto the absorption of about 55% of the pump. The peak fiber heating,which occurs along the extent of the circular first cladding regioncross section, is at 1.5 m, which corresponds to about 15% pump powerabsorption. 10% of the input pump power is absorbed at a fiber positionof about 1.1 m.

Considering further the impact of the selection of a particular sitelocation along fiber length for a change between low and highpump-absorption cross-sectional geometry for the first cladding region,consider the plot of FIG. 8. This plot refers to conditions in whichinput pump light is launched into the first cladding region with a pumppower of 2000 W at a wavelength of 976 nm and a co-propagating inputsignal power of 10 W at a wavelength of 1064 nm. The fiber has a corethat is ytterbium-doped, having an absorption coefficient of 0.46dB/meter at 915 nm, with a 25 μm-diameter core. The first claddingregion is 400 μm in diameter, for both circular and octagonalgeometries.

Each data point in the lower line plotted in FIG. 8 represents the peakheat dissipated along a portion of a fiber having a circularcross-sectional cladding, as a function of the distance, in meters,along the fiber at which the first cladding region cross section changesfrom circular to octagonal. Each data point plotted in the upper linerepresents the peak heat dissipated along a portion of a fiber having anoctagonal cross-sectional cladding, also as a function of the distance,in meters, along the fiber at which the first cladding region crosssection changes from circular to octagonal. The plot of FIG. 8 isparticularly useful for analyzing a given fiber system to determine anoptimum design of the first cladding region geometry. A suitablecommercial mathematical simulation program can be employed to achievemeaningful simulation results, as described above.

FIG. 9 is a plot of the pump light power, the signal light power, andthe heat deposited in the fiber from the core, along fiber length, for afiber having a change in cross-sectional shape of the first claddingregion at the site where about 10% of the pump power absorption hasoccurred, which is 1.0 meters. The conditions of this fiber operationare that the input pump light is launched into the first cladding regionwith a pump power of 2000 W at a wavelength of 976 nm and an inputsignal power of 10 W at a wavelength of 1064 nm. The fiber has a corethat is ytterbium-doped, at about the same concentration across theentire length of the 10 m fiber with a 25 μm-diameter core, with a 25μm-diameter core, and a 400 μm-diameter first cladding region.

As shown in this plot, the peak heating of the fiber is about 38 W/m atjust before 2 m along the fiber length. This is nearly identical to thepeak fiber heating that occurs in a fiber having an all-octagonal firstcladding region along its entire length. This result demonstrates thatfor many applications there may not be much advantage to set the shapechange location at the position where about 10% or less the pump powerabsorption occurs.

FIG. 10 is a plot of the pump light power, the signal light power, andthe heat deposited in the fiber from the core, along the fiber length,for a fiber having a gradual change in cross-sectional shape of thefirst cladding region. The first cladding region has circular a crosssection from the first end of the doped fiber to a site at about 3.5 mfrom the end of the doped fiber, with a first cladding absorptioncoefficient of about 0.23 dB/meter at 915 nm. For positions along thefiber length between 3.5 m and 7.5 m, the cross-sectional geometry ofthe first cladding layer is gradually changed from circular tooctagonal, having a first cladding absorption coefficient that changesfrom about 0.23 dB/meter to 0.46 dB/meter at 915 nm. The cross-sectionalgeometry change ends at 7.5 m from the end of the doped fiber. Thus, thecross section of the first cladding region is circular from the firstend of the fiber to 3.5 m, then transitions to octagonal between 3.5 mand 7.5 m, and then is octagonal from 7.5 m onward, with a firstcladding absorption coefficient of about 0.46 dB/meter at 915 nm. Thepeak heating is about 24 W/m and is shown in the plot to occur at bothabout 2 m and about 3.5 m along the fiber length.

At 3.5 m along the fiber length, where the cross-sectional geometrytransitions begins, there has been a 40% absorption of pump power. Thisgeometric transition point at 3.5 m causes a new heat peak at about 4 m,which is about the same as the heat peak occurring in an all-circularfirst cladding region at a position of about 2 m. With this similar heatpeak, the fiber material is likely to be at the same failure level inboth locations and so is shown to be somewhat optimum. If one heat peakwas lower or higher than the other, then it would be indicated that theoverall length of the fiber was longer than it could be optimized for.For example, if the geometry transition was started at a fiber positionmuch greater than 3.5 m, then the second heat peak would be lower thanthe 24 W/m level of the heat peak at about 2 m and the overall fiberlength would have to be longer than optimum because the circular firstsection of fiber is less efficient at pump absorption. If the geometrytransition started between about 2 m and about 3.5 m, then the secondheat peak would be higher than the 24 W/m of the first peak at about 2 mand that would be a new hot spot in the fiber, which could causedetrimental operational issues. If the geometry transition started atless than 2 m, the fiber operation would be very similar to the case ofa fiber having an all-octagonal first cladding region and the first heatpeak would be much great greater than 24 W/m, which is the unwantedcondition of conventional doped fiber.

FIG. 11 is a plot of the pump light power, the signal light power, andthe heat deposited in the fiber from the core, along fiber length, for afiber having a change in cross-sectional shape of the first claddingregion at the fiber position where about 20% of the pump powerabsorption has been achieved. This plot refers to conditions in whichthe input pump light is launched into the first cladding region with apump power of 2000 W at a wavelength of 976 nm and an input signal powerof 10 W at a wavelength of 1064 nm. The fiber has a core that isytterbium-doped, at about the same concentration across the entirelength of the 10 m fiber with a 25 μm-diameter core, and a 400μm-diameter first cladding region.

The first cladding region of this fiber has a circular cross-sectionalgeometry for fiber positions from 0.0 m to 2.0 m, with a first claddingabsorption coefficient of about 0.23 dB/meter at 915 nm, and the firstcladding region has an octagonal cross section for fiber sites from 2.0m along the fiber onward, with a first cladding absorption coefficientof about 0.46 dB/meter at 915 nm. The plot shows that the peak heatingin the fiber reaches about 38 W/m at 2 m along the fiber length. This isnot any improvement over a fiber having a first cladding region that isoctagonal for the entire fiber, and shows that positioning of the sitefor change in first cladding region shape at the fiber position whereabout 20% of the pump power has been absorbed, may not useful for manyapplications. This demonstrates that there exists a wide range ofpossible sites for change in cross-sectional geometry but not allpossible sites may be optimal.

FIG. 12 is a plot of the pump light power, the signal light power, andthe heat deposited in the fiber from the core, along fiber length, for afiber having a change in cross-sectional shape of the first claddingregion at the fiber position where about 30% of the pump power has beenabsorbed. This plot refers to conditions in which the input pump lightis launched into the first cladding region with a pump power of 2000 Wat a wavelength of 976 nm and an input signal power of 10 W at awavelength of 1064 nm. The fiber has a core that is ytterbium-doped, atabout the same concentration across the entire length of the fiber witha 25 μm-diameter core, and a 400 μm-diameter first cladding region.

The first cladding region of this fiber has a circular cross-sectionalgeometry for fiber positions from 0.0 m to 3.0 m, with a first claddingabsorption coefficient of about 0.23 dB/meter at 915 nm, and the firstcladding region has an octagonal cross section for fiber sites from 3.0m along the fiber onward (with a first cladding absorption coefficientof about 0.46 dB/meter at 915 nm). The plot shows that the peak heatingin the fiber reaches about 36 W/m at 2 m along the fiber length. This isa small but measurable improvement over a fiber having a first claddingregion that is octagonal for the entire fiber, and shows thatpositioning of the site for change in first cladding regioncross-sectional geometry at the fiber position where about 30% of theinput pump power has been absorbed can be useful.

This analysis and the data plots described above are not meant to belimiting and are provided as exemplary embodiments that demonstrate adoped fiber having a first cladding region that is of a relativelylow-absorption, circular, cross section for a low-absorption extent ofthe fiber and that is of a relatively high-absorption, octagonal, crosssection for a high-absorption extent of the fiber. FIG. 8 demonstratedin particular that there exists an inflection point, at 20% of effectivedoped fiber length, for which a distinction in effectiveness of shapechange is found. For other non-circular, high-absorption cross-sectionalshapes, such as a circle with a flat or other nonsymmetrical feature,such an inflection point in data can be determined, and should beexpected to be different than that data shown for thecircular-to-octagonal shape change of this particular embodiment.

This analysis is specific to the section or sections of a fiber systemthat are doped. It is recognized that many composite fiber systems canbe employed in which fiber splicing and other arrangements enable manydifferent combinations that included undoped as well as doped fiber.FIG. 13A is a schematic diagram of an end-pumped co-propagating fiberamplifier configuration including a combiner for combining signal lightand pump light that are together input to a passive, undoped fiberlength that is spliced to an active, doped fiber length. An undoped coreis therefore included here in the cladding, and a high-absorptiongeometry can be employed for the first cladding region along the undopedfiber length. FIG. 13B is a schematic diagram of an end-pumpedcounter-propagating fiber amplifier configuration including a combinerfor combining counter-propagating signal light and pump light. A dopedactive fiber length is arranged between a first passive fiber length anda second passive fiber length. FIG. 13C is a schematic diagram of anend-pumped co- and counter-propagating fiber amplifier configurationincluding combiners at ends of the fiber arrangements. The active, dopedfiber section here is arranged between several sections of passivefiber. FIG. 13D is a schematic diagram of a co-propagating fiberamplifier configuration in which a pump fiber provides the pump lightinto a doped fiber section and a passive fiber section provides theinput signal light, with the passive fiber section being spliced intothe doped active fiber section. FIG. 13E is a schematic diagram of aco-propagating fiber amplifier configuration including several pumplight input sites along an active, doped fiber section. The signal lightis input through a passive fiber length to one end of the doped fiberlength. The pump fibers are integrated with the doped fiber section atselected sites.

All of the arrangements shown in FIGS. 13A-13E represent severalembodiments in a wide range of embodiments that can be employed with anactive doped fiber in the design and manufacture of fiber lasers andamplifiers. The cross-sectional geometry of the first cladding region ischanged at one or more points along the doped fiber length or lengths ofthe system, and the location of changes in cross-sectional geometry arecomputed based on the doped fiber length and the effective doped fiberlength, not the undoped fiber sections that may be included in the fibersystem.

In production of the fiber provided herein, the two or morecross-sectional geometries that are selected for the first claddingregion can be imposed on the first cladding region as the fiber isdrawn, or can be achieved in a post-draw process. Referring to FIGS.14A-14B, in a first embodiment, a fiber draw tower includes shapinghardware that is arranged to produce selected cross-sectional geometryat selected sites along the length of the first cladding region. Asshown in FIGS. 14A-14B, such shaping hardware can be provided as, e.g.,a heating element that in a selected manner causes the first claddingregion to be shaped by thermal control. The fiber preform to be drawninto a fiber is configured based on this control so that the resultingfiber exhibits the desired cross-sectional profile. For example, anall-octagonal first cladding region can be provided in the fiber preformand the shaping hardware employed to provide a circular cross section atselected portions of the fiber.

In a process for the thermal shaping of the first cladding region of thefiber, a radiative or conductive heat source can be employed thatsurrounds the circumference of the fiber. If the surface of the outersecond cladding is melted to pull, by surface tension, the firstcladding region from an octagonal or similar geometry to a circulargeometry before hardening of the outer cladding region, then a change ingeometry of the first cladding region can be obtained. Here it can bepreferred that the heat source application is of relatively shortduration so that the doped fiber core does not deform or suffersignificant thermal diffusion, which could affect the modal propertiesof the fiber. A radiative source can be controlled to operate at aselected wavelength that is strongly absorbed by the outer secondcladding region at or near the surface of this region to assist inpreferential melting of the surface layers of the fiber withoutdeformation of the fiber core. It can be preferred to modulate the heatsource intensity on the scale of about 1 m or less of the fiber lengthto obtain the desired cross-sectional geometry profile.

In a further embodiment, a radiative or conductive heat source iscontrolled to unevenly heat the fiber around the circumference of thefiber, e.g., being biased to preferentially heat a subsection of thecircumference of the fiber. In this embodiment, a fiber preform in whichthe first cladding region has circular symmetry can be processed tobreak the symmetry by generating a flat or otherwise deformed sitearound the circumference of the first cladding region. Pulsing of alaser that is focused on one site around the fiber circumference canhere be employed to effectively generate a defect or other localizedchange in geometry. This thermal shaping process can be assisted withthe use of fiber tension, axial twist, directed gas flow, directedpressure application, or other assisting technique. The heat source andassistance technique is modulated on the scale of about 1 m or less forthe desired tailoring of the cross-sectional profile.

The draw tower shaping hardware for enabling the shaping of the firstcladding region can include shaping sensors, such as temperaturesensors, laser micrometers, and other sensors. Closed-loop feedbackcontrol can therefore be implemented to control the shaping of the firstcladding region cross-sectional geometry. But open-loop shaping of thefirst cladding region can be conducted where shaping control iswell-characterized.

In a further embodiment for shaping the cross-sectional geometry of thefirst cladding region, the fiber preform is shaped specifically tocooperate with selected a draw process control for effecting changes inthe first cladding region cross-sectional geometry. For example, thepreform can be provided with a photonic/air-like fiber structure in thepreform section of the first cladding region, with the preform structurebeing selected to break the symmetry of the first cladding region duringthe fiber draw such that a high-pump absorption effect is achieved. Thisstructure can be provided along the length of the preform by, e.g.,drilling or using, e.g., capillary-like glass tubes. During draw of thepreform on the fiber draw tower, the voids are selectively eithermaintained or collapsed by the thermal shaping process hardware to formcircular symmetry at selected portions of the fiber. The voids can bepreferentially sited near the surface of the fiber so that the heatsource can effectively cause collapse without disturbing the core of thefiber. The heat source and assistance technique is modulated on thescale of about 1 m or less for the desired tailoring of thecross-sectional profile.

If desired, the first cladding region can be shaped prior to formationof the second outer cladding region around the first cladding region. Inpreferred embodiments, sites around the circumference of the firstcladding region are shaved, abraded, thermally softened and then shaped,mechanically shaped and then annealed, or otherwise processed to produceselected cross-sectional geometry of the first cladding region prior toformation of the second cladding region.

The thermal shaping techniques described above can be employed afterthermal drawing rather than during the fiber draw itself. Thus, thefiber draw tower can include shaping apparatus, or alternatively,post-draw apparatus can be employed for shaping fiber after the draw.The outer second cladding region can be removed from portions of thefiber to expose the first cladding region for shaping those portions ofthe first cladding region, or processes that adjust the first claddingregion with the second cladding region in place can be employed afterthe fiber is drawn. Here a long length of fiber can be processed andthen cut into lengths so that each length has the desired change incross-sectional shape at a selected site or sites relative to the endsof each cut fiber length.

Whatever shaping mechanism is employed, whether during fiber draw orafter fiber draw, it is preferable that the dimensions andcross-sectional geometry of the fiber core not be changed or be changedonly very minimally over the entire length of the fiber. In oneembodiment, the dimensions and cross-sectional geometry of the second,outer cladding region, radially outside of the first cladding region,are also not changed over the entire length of the fiber. In onepreferred embodiment, the cross section of the first cladding region ischanged between at least two different geometries while thecross-sectional geometry of the fiber core and the second claddingregion remain constant along the entire fiber length.

The fiber provided herein can be manufactured of any suitable materials.For example, the fiber core and first cladding region can be provided assuitable materials such as silica-based glasses, with dopantsincorporated and selected to change the refractive indexes, such as Aland fluoride, and also dopants to act as the active laser medium, e.g.,Yb. The second, outer cladding region can be any suitable material,e.g., glass, silicones, (fluoro)acrylates and any other organiccompounds with desired optical and thermal properties. Other glassmaterials such as fluoride, ZBLAN, etc., can also be employed.

With this description it is shown that the heating of an active dopedfiber is reduced by limiting the degree of pump light absorption fromthe first cladding region into the fiber core through control of thefirst cladding region cross-sectional geometry. The first claddingregion cross-sectional geometry is selectively changed, as either agradual change or an abrupt change, along the length of the fiber, toprovide a high-absorption length of the fiber and to provide alow-absorption length of the fiber. The reduction in heat that thisdesign enables provides for superior operation of pulsed fiber lasersand amplifiers in which the fiber length is preferably as short aspossible. Applications in materials processing, medicine, e.g., lasersurgery, LIDAR, communications, and other fields are enabled by thedesign. In addition, single mode lasers and amplifiers, pulsed Quasi-CW,and CW, all in which it is preferable to minimize fiber length, are alsowell-addressed. Material processing applications are also welladdressed, for example, for high power, Yb-doped industrial lasers andamplifiers, often exceeding 1 kW in average power, and for high powerlasers and amplifiers doped with other dopants, such as erbium. In allof these applications, the reduction in heat dissipation achieved by thefiber design provided herein enables fiber operation that far surpassesthat of conventional fiber configurations.

It is recognized that those skilled in the art may make variousmodifications and additions to the embodiments described above withoutdeparting from the spirit and scope of the present contribution to theart. Accordingly, it is to be understood that the protection sought tobe afforded hereby should be deemed to extend to the subject matterclaims and all equivalents thereof fairly within the scope of theinvention.

I claim:
 1. A continuous fiber length comprising: a doped fiber core extending over a continuous fiber length; a first cladding region radially surrounding the doped fiber core over the fiber length; a second cladding region radially surrounding the first cladding region over the fiber length; at least one pump light input site arranged to accept input pump light into the first cladding region; the continuous fiber length including a low-absorption length over which the first cladding region comprises a first cross-sectional geometry that produces a first level of absorption of input pump light from the first cladding region to the doped fiber core, the low-absorption length extending from about a pump light input site for an extent along the fiber length over which the doped fiber core can absorb at least about 10% of input pump light from the first cladding region; and the continuous fiber length including a high-absorption length over which the first cladding region comprises a second cross-sectional geometry that produces a second level of absorption of input pump light from the first cladding region to the doped fiber core, the second level of absorption of input pump light being greater than the first level of absorption of input pump light.
 2. The continuous fiber length of claim 1 wherein the low-absorption length of the fiber extends from about a pump light input site for an extent along the fiber length over which the doped fiber core can absorb at least about 20% of input pump light from the first cladding region.
 3. The continuous fiber length of claim 1 wherein the low-absorption length of the fiber extends from about a pump light input site for an extent along the fiber length over which the doped fiber core can absorb at least about 30% of input pump light from the first cladding region.
 4. The continuous fiber length of claim 1 wherein the low-absorption length of the fiber extends from about a pump light input site for an extent along the fiber length over which the doped fiber core can absorb at least between about 10% and about 40% of input pump light from the first cladding region.
 5. The continuous fiber length of claim 1 wherein the low-absorption length of the fiber extends from about a pump input site that is at one end of the fiber length.
 6. The continuous fiber length of claim 1 wherein the low-absorption length of the fiber comprises a first low-absorption fiber length that extends from about a pump input site that is at one end of the fiber length, and further comprising a second low-absorption length of the fiber that extends from a second pump input site that is at a second end of the fiber.
 7. The continuous fiber length of claim 1 wherein the low-absorption length of the fiber extends from about a pump input site that is disposed at a site along the fiber length, between ends of the fiber.
 8. The continuous fiber length of claim 1 wherein the second cladding region has a substantially constant outer diameter along the fiber length.
 9. The continuous fiber length of claim 1 wherein the first cross-sectional geometry is generally circular.
 10. The continuous fiber length of claim 1 wherein the second cross-sectional geometry consists of a geometry selected from the group consisting of octagonal, pentagonal, and hexagonal.
 11. The continuous fiber length of claim 1 wherein the second cross-sectional geometry is a circular geometry and includes at least one flat perimeter section in the circular cross-sectional geometry.
 12. The continuous fiber length of claim 1 wherein the second cross-sectional geometry is a circular geometry and includes at least one notch into the circular cross-sectional geometry at a perimeter of the circular cross-sectional geometry.
 13. The continuous fiber length of claim 1 wherein the second cross-sectional geometry is a circular geometry and includes at least one hole in the circular cross-sectional geometry.
 14. The continuous fiber length of claim 1 further comprising a transition length over which the first cross-sectional geometry of the first cladding region gradually changes to the second cross-sectional geometry of the first cladding region.
 15. The continuous fiber length of claim 1 further comprising a transition site, at an end of the low-absorption length, at which the first cross-sectional geometry of the first cladding region is changed to the second cross-sectional geometry of the first cladding region.
 16. The continuous fiber length of claim 1 wherein the doped fiber core has a diameter and a fiber core cross-sectional geometry that is substantially unchanged over the fiber length.
 17. The continuous fiber length of claim 1 further comprising an undoped fiber core, connected to the doped fiber core, having an undoped fiber core length along the fiber length, the first and second cladding regions radially surrounding the undoped fiber core over the undoped fiber core length; and wherein the second cross-sectional geometry of the first cladding region extends over the undoped fiber core length.
 18. The continuous fiber length of claim 1 wherein the doped fiber core comprises a glass selected from the group consisting of silica-based glasses and fluoride-based glasses; and wherein the doped fiber core is doped with a dopant selected from the group consisting of ytterbium, erbium, praseodymium, neodymium, and tellurium.
 19. A continuous fiber length comprising: a doped fiber core extending over a continuous fiber length; a first cladding region radially surrounding the doped fiber core over the fiber length; a second cladding region radially surrounding the first cladding region over the fiber length; at least one pump light input site arranged to accept input pump light into the first cladding region; the continuous fiber length including a low-absorption length over which the first cladding region comprises a first cross-sectional geometry that produces a first level of absorption of input pump light from the first cladding region to the doped fiber core, the low-absorption length extending from about a site along the fiber length at which at least about 30% of input pump light can be absorbed from the first cladding region to the doped fiber core to a site along the fiber length at which no more than about 80% of input pump light can be been absorbed from the first cladding region to the doped fiber core; and the continuous fiber length including a high-absorption length over which the first cladding region comprises a second cross-sectional geometry that produces a second level of absorption of input pump light from the first cladding region to the doped fiber core, the second level of absorption of input pump light being greater than the first level of absorption of input pump light.
 20. The continuous fiber length of claim 19 wherein the low-absorption length extends from a site along the fiber length at which at least about 30% of input pump light can be absorbed from the first cladding region to the doped fiber core to a site along the fiber length at which no more than about 90% of input pump light can be been absorbed from the first cladding region to the doped fiber core. 